Advisers manning the “Frank” helpline are informing callers they believed to be children as young as 13 that alcohol is a “much more powerful drug than cannabis” and that using the illegal drug recreationally is not harmful because it “doesn’t get you that high”.

Callers are also being told that taking ecstasy will not lead to long-term damage and that if they are in doubt, to “just take half a pill and if you are handling that OK, you can take the other half.”

They are even being told that they would be able to smoke a cannabis joint, on top of ecstasy, with no ill-effects. The advice, given to reporters who rang the helpline posing as young people, has alarmed anti-drugs campaigners who branded it “scandalous” and “irresponsible.”

Health experts have condemned the advice given to children as “frankly appalling”, “factually incorrect” and “worryingly cavalier”.

After being presented with the findings, the Government last night said it had launched an immediate investigation into the Frank service, which is funded by three separate departments, and said it would be taking action on advisers involved.

Chris Grayling, the shadow Home Secretary, said: “The idea that the Government’s helpline should be saying to young people “go for it” and that cannabis should be class C when it has just been classified by the Government as class B, shows that the Home Office is all over the place in its approach to drugs.”

Professor Neil McKeganey, professor of drug misuse research, at Glasgow University, said: “Having read one of the transcripts, it is extraordinary that the Frank councillor seems more concerned to place cannabis smoking in some kind of comfort zone of acceptable behaviour rather than address the risks of such drug use on the part of a 13-year-old child.”

Mary Brett, a spokesman for the Talking About Cannabis charity, said: “It is scandalous. These people are talking to kids, for goodness sake. Taking drugs can trigger all kinds of psychosis in people that have a genetic predisposition to it. Why are they not told that? Medical experts have said time and again that skunk, the newer type of cannabis that many young people are taking, is dangerous.

“These children are being told they can choose. But the risky bit of their brains develops before the inhibitory bit of their brain and they take risks. “They have to be told 'this is not for you’. When they hear fair, reasoned arguments against, they respond. It is obvious they are not hearing them from Frank.”

The helpline, established by the Government in 2003 with £3 million funding, was described in a Home Office drugs strategy recently as “the key channel by which Government communicates the dangers of drugs, including cannabis, to young people”.

But in calls to its helpline, manned 24 hours a day, seven days a week, reporters posing as teenagers were told by different advisers that drug taking was not harmful.

At no point in the conversations did the Frank team try to dissuade the callers from taking drugs. The effects on the body were played down to the extent that one adviser, referring to ecstasy, said: “At the end of the day I know where you’re coming from – doing a pill and it felt great.”

Another councillor said that cannabis, a class B drug, should be regarded as class C and that “cannabis doesn’t really get you that high. You know you are always in control”.

A third adviser stated: “nicotine is physically addictive. Cannabis isn’t. You can stop smoking it any time you want.”

Alcohol was presented as a much greater danger than illegal drugs, including heroin, more expensive and with many more negative effects. One adviser told a caller: “The withdrawals of alcohol are worse than heroine for example; people can die when they become addicted to alcohol and stop suddenly.”

The reporters were also told that the police “would not do anything” if they found a young person with cannabis and that if they are caught with pills, they should say they were for their own use to avoid being prosecuted as a dealer.

In one call, where the reporter claimed to be the friend of a 13-year-old boy who had started smoking cannabis, the adviser said: “He won’t get addicted, no. Tell him you spoke to Frank and they told me it’s not as dangerous as alcohol. Tell him they said by using it recreationally, it’s not as bad as alcohol, because that’s the truth in terms of the power of the drug.”

He went on to say that if alcohol was illegal, it would be a class A drug, the most harmful category, whereas “cannabis should just be a class C drug”.

Another reporter, posing as a 15-year-old girl who had taken her first ecstasy tablet, asked if it would affect her health in any way. The response was “Nah”. He told the caller that he could not say “go and take Es, you’re absolutely fine”, but that “in terms of taking a pill like that, it’s not going to affect your health”.

He went on to say “obviously you had a really good experience. It’s like most things, if you do it in moderation, you lessen your chances.

“A good idea is if you don’t know what it is you are taking, take a half a one and see how you go and if you are handling that OK, you can take the other half.”

The adviser was also unsure what classification the Class A drug was.

During a discussion where the adviser talked about mixing drugs, the reporter asked if it was safe to have cannabis after taking an ecstasy pill. The adviser said: “Again, I’m not condoning it but it wouldn’t spin you out like another pill or powder. If you’re asking me if you could have a spliff with it, would it have any major affects, generally speaking, no, although people are individuals so what works for one might not work for another, but generally speaking, no, you’d be able to have spliff with it.”

An estimated five million people in the UK are users of illegal or street drugs.

Health experts are growing increasingly worried about the affects on young people’s mental health. There is also growing evidence that contrary to earlier assumptions, cannabis can be addictive.

Varieties of skunk, which contain much higher levels of tetrahydrocannabinol (THC), the active chemical, are more dangerous than the cannabis used in the 1960s and 1970s but are now widespread and often the choice of young people.

Dr Zerrin Atakan, consultant psychiatrist at the Institute of Psychiatry, said: “Any drug use while the brain is still developing may lead to structural or functional changes. One Australian study has shown that heavy cannabis users show clear structural abnormalities of the brain.

“Another recent study has also shown that cannabis use before 18 can lead to abnormalities in areas of the brain that control memory, attention, decision-making and language skills.

“Also, contrary to previously held beliefs, it is now considered that regular users can develop 'tolerance’ to the drug, one of the main characteristics of addiction. Regular users require higher doses to become 'stoned’. Some people find it very hard to give it up and become highly anxious if they do.”

According to the Home Office, drug use among all ages, including young people, has fallen in recent years. The Government, which downgraded cannabis to a grade C drug in 2004, has recently reclassified it to B.

A Government spokesman said: “It is completely unacceptable for a Frank adviser to be giving out wrong, misleading and inaccurate information. We are urgently looking into the matter and will identify the person or persons involved and take action.

“Frank is an important resource for young people who need help and advice about drugs. It is vital that Frank advisers give out correct and straight forward advice – we have therefore commissioned a review of the training advisers receive and will act upon it.”